Figure 1. Excitonic insulator and Ta₂NiSe₅ structure.

(a) Schematic phase diagram of excitonic gap formation as a function of E_G. The dotted line indicates the crossover from semimetallic to semiconducting behaviour. The insets show the relation between E_G and Δ_E for the metallic and insulating limits. E_G is the one-electron bandgap between conduction (red) and valence band (blue). The Coulomb interaction between electron- and hole-like excitations can lead to electron–hole pairing (exciton formation) and the opening of a new many-body excitation gap Δ_E in the excitonic insulator phase. (b) The orthorhombic phase of Ta₂NiSe₅ shown in two orientations emphasizing the layered nature in the a–c plane, and (c) the TaSe₆ and NiSe₄ chains along the a direction (right).